Amplitude modulation of signals involves multiplying a signal Vs*sin(2πfSIGNALt) and a carrier sin(2πfCARRIERt). The carrier term is unitless and not nominally a voltage. An analog multiplier may be used to form Vs*sin(2πfSIGNALt)*sin(2πfCARRIERt). This results in modulated terms Vs/2[cos(2π(fCARRIER−fSIGNAL)t)−cos(2π(fCARRIER+fSIGNAL)t)]. If the signal is centered at ground (or some ground-like reference) the result is a double-sideband, suppressed-carrier signal. If the signal is centered at some DC voltage, then there is an additional carrier term and the result is amplitude modulation as for example used with AM radio. This approach is not commonly used because analog multipliers at common frequencies are difficult to construct.
One known method of making a suppressed-carrier AM modulator employs a ring modulator (FIG. 1). The signal (Signal In), carrier local oscillator (Carrier LO), and output RF Out are transformer coupled (e.g., via input and output windings 103 and 105, respectively), resulting in zero DC in the output RF Out. There is suppression of carrier terms, but not carrier harmonics. The ring diodes 104 act non-linearly to multiply signal and carrier, resulting in generation of output signals at odd multiples of the carrier.
Another known method of producing AM modulators in the switched current circuit (FIG. 2). This circuit consists of an upper quad current switch 202 supplied by a voltage source 206, and driven by a lower differential amplifier 204 emitter coupled to a current source (Ibias). A full-wave balanced multiplication of the two input voltages (Carrier and Signal In) occurs. That is, the output signal RF Out is a constant times the product of the two input signals Carrier LO and Signal In. This circuit is typically terminated with a differential transformer or resistive pull-ups and a differential amplifier. DC bias on one or other of the signal inputs results in lack of suppression of the carrier. If the signal input is transformer-coupled and suitably biased, the amount of carrier feed-through is determined by the mismatch of the signal transistor pair (204), represented by an offset voltage. The upper transistor pairs 202 are switching devices, not analog-multiplying devices. Thus, they do not multiply by a pure carrier signal, but rather by a square wave. In the same way as the ring modulator (FIG. 1), the multiplication of the signal with a switching carrier results in alias terms at odd multiples of the carrier.
An analog CMOS switching circuit is illustrated in FIG. 3. When the carrier is high, the switch 306 is closed and the amplifier 302 multiplies by −1. When the carrier is low, the switch 306 is open and the amplifier 306 multiplies the input Vin by +1 to produce the modulated output signal Vmod-out. Like the ring modulator and the current-switched (bipolar) circuit, this switched amplifier results in carrier terms at odd multiples of the carrier frequency. The resistors 304 may be balanced (i.e., substantially equal in resistive value). Example carrier and modulated output signals in both the time and frequency domains are illustrated in FIGS. 4 and 5.
A disadvantage of all of the previous methods is generation of replicas of the modulated signal at harmonics of the carrier. The output of the modulator has desired components at the carrier plus and minus the signal frequency, then at carrier harmonics plus and minus the signal frequency until the circuit runs out of bandwidth. The carrier is represented by the series given by (1) in Table 1, below. The balanced modulator outputs a signal approximated by the product given in (2) of Table 1. The level of the harmonics is proportional to the inverse of the harmonic number. The third harmonic level is ⅓ times the fundamental or −10 dB. The fifth harmonic level is ⅕ of the fundamental or −14 dB, and so on (see for example FIG. 5). In order to significantly reduce unwanted signals spilling over into adjacent bands, these harmonics must be significantly attenuated. In RF circuits, this is done with filtering. In the case of the ring modulator (FIG. 1), the output transformer may be tuned. In the case of the current switched circuit (FIG. 2) or the voltage switched circuit (FIG. 3), this means that an additional filter stage is required. This may necessitate the use of additional active and passive components or a complex passive (tuned LC) filter.